Hard disc drives are commonly used in a multitude of computer environments ranging from super computers through notebook computers to store large amounts of data in a form that can be made readily available to a user. Typically, a disc drive comprises one or more magnetic discs that are rotated by a spindle motor at a constant high speed. The surface of each disc is a data recording surface divided into a series of generally concentric recording tracks radially spaced across a band having an inner diameter and an outer diameter. Extending around the discs, the data tracks store data within the radial extent of the tracks on the disc surfaces in the form of magnetic flux transitions induced by an array of transducers, commonly called read/write heads. Typically, each data track is divided into a number of data sectors that store fixed sized data blocks.
The read/write head includes an interactive element such as a magnetic transducer which senses the magnetic transitions on a selected data track to read the data stored on the track. Alternatively, the read/write head transmits an electrical signal that induces magnetic transitions on the selected data track to write data to the track.
As is known in the art, each read/write head is mounted to a load arm that is supported by an actuator arm and is selectively positionable by a rotary actuator assembly over a selected data track of the disc to either read data from or write data to the selected data track. The read/write head includes a slider assembly having an air-bearing surface that causes the read/write head to fly above the data tracks of the disc surface. The air-bearing is developed as a result of load forces applied to the read/write head by the load arm interacting with air currents, produced by rotation of the disc.
Typically, a plurality of open-center discs and spacer rings are alternately stacked on the hub of a spindle motor. The hub, defining the core of the stack, serves to align the discs and spacer rings around a common centerline. Movement of the discs and spacer rings is typically constrained by placing the stack under a compressive load and maintaining the load by means of a clamp ring. Collectively the discs, spacer rings, clamp ring and spindle motor define a disc stack envelope or disc pack. The read/write heads are mounted on a complementary stack of actuator arms which comprise an actuator assembly which accesses the surfaces of the stacked discs of the disc pack. The actuator assembly generally includes a precision component, known to in the art as an "E-block," which provides a mounting for the voice coil, the load arms for the read/write heads, the flex circuit assembly, the cartridge bearing assembly and the read/write head wires.
The read/write head wires conduct electrical signals from the read/write heads to a flex circuit which conducts the electrical signals to a flex circuit connector. The flex circuit connector in turn is mounted to a flex circuit mounting bracket, and the mounting bracket is mounted to a disc drive basedeck. External to the basedeck the flex circuit connector is secured to a printed circuit board assembly (PCB). For a general discussion of actuator assembly techniques, see U.S. Pat. No. 5,404,636 entitled METHOD OF ASSEMBLING A DISC DRIVE ACTUATOR, issued to Stefansky et al., assigned to the assignee of the present invention.
The head-disc assembly (HDA) of a disc drive is typically assembled in a clean room environment. The need for maintaining a clean room environment (free of contaminants of 0.3 micron and larger) is to ensure that the head-disc interface remains unencumbered and damage free. The slightest damage to the surface of a disc or read/write head can result in a catastrophic failure of the disc drive. The primary causes of catastrophic failure, particularly read/write head crashes (a non-recoverable, catastrophic failure of the disc drive), are generally characterized as contamination, exposure to mechanically induced shock and non-shock induced damage. One common cause of non-shock induced damage is typically traced to the assembly process and generally stems from handling damage sustained by the HDA during the assembly process. Another cause of non-shock induced damage results from an inability to achieve the proper placement of the E-block relative to the datum of the cartridge bearing assembly. In many disc drives the cartridge bearing assembly's datum is the defining datum from which the balance of a disc drives mechanical components draw their reference.
Several factors that bear particularly on the problem of assembly process induced damage and misalignment are the physical size of the disc drive, the spacing of the components, the recording densities sought to be achieved and the level of precision to be maintained during the assembly process. The high levels of precision required by the assembly process are necessary to attain the operational tolerances required by the disc drive. The rigorous operational tolerances are in response to market demands that have driven the need to decrease the physical size of the disc drive, while simultaneously increasing the disc drive storage capacity and performance characteristics.
Demands on the disc drive mechanical components and assembly procedures have become increasingly more critical in order to support capability and size reduction in the face of the new market demands. Part-to-part variation in critical functional attributes in the magnitude of a micro-inch can result in disc drive failures. These factors are of particular importance when assembling the Actuator assembly. Misalignment of the E-block relative to the cartridge bearing assembly datum or interference between the cartridge bearing assembly and the E-block, to a level beyond the ability of the disc electronics to accommodate the variance, will result in disc drive failure.
Additionally, as disc drive designs continue to decrease in size, smaller read/write heads, thinner substrates, longer and thinner actuator arms and thinner gimbal assemblies continue to be incorporated into the drives. This trend significantly increases the need to improve HDA assembly processes and develop new mechanical designs conducive to producing consistent product as a natural outcome of executing the assembly process steps.
The progression of continually decreasing disc thickness and disc spacing, together with increasing track density and increasing numbers of discs in the disc pack, has resulted in a demand for tools, methods and robust designs of ever increasing sophistication. One result of the growth in demand for sophisticated HDA assembly processes has been a demand for more compliant mechanical designs that involve fewer critical alignments, while achieving ever-increasing levels of precision.
In addition to the difficulties faced in assembling, high capacity, complex disc drives, actual product performance requirements have dictated the need to develop new mechanical interfaces to ensure compliance with operating specifications. The primary factor driving more stringent demands on the mechanical components and HDA assembly process is the continually increasing areal densities and data transfer rates of the drives.
The continuing trend in the disc drive industry is to develop products with ever increasing areal densities, decreasing access times and increasing rotational disc pack speeds. These factors place greater demands on the ability of servo systems to control the position of read/write heads relative to the data tracks. This same trend makes disc drives much more susceptible to detrimental resonance frequencies, fly height variations of the read/write heads, overshoot of the read/write heads during seek operations and read/write head oscillation (collectively referred to as mechanically induced noise and instability).
By design, a disc drive typically has a discreet threshold level of resistance to withstand mechanically induced noise and instability, below which the servo system is not impaired and the read/write channel is able to function. The servo system can accommodate a finite level of detrimental resonance frequencies, overshoot of the read/write heads during seek operations or read/write head oscillation. The read/write channel can accommodate a less than nominal signal resulting from fly height variations of the read/write heads and discreet level of read/write head oscillation. The mechanical interface integrity between the E-block and cartridge bearing assembly of the actuator assembly is highly critical in minimizing and maintaining a low level of mechanically induced noise and instability that affects the servo system and read/write channel. Additionally, assuring mechanical interface integrity between the E-block and the cartridge bearing assembly in the form of mechanical alignment is essential in achieving the load forces required by the read/write head to ensure proper fly height for data exchanges.
The primary manifestations of mechanically induced noise and instability are: seek induced read/write head oscillation; resonance induced read/write head oscillation; resonance of the actuator at the servo systems crossover frequency; overshoot; and externally induced vibrations. Of these manifestations of mechanically induced noise and instability, read/write head oscillations, and externally induced vibrations are particularly pertinent to the present invention.
Read/write head oscillations are often introduced to the system via deformations or irregularities in the topology of the disc surface; harmonics induced into the actuator assembly by the rotating disc pack; and the resonance response of the actuator assembly to the levels of energy injected into the system. Relatively high levels of energy for a short duration of time are encountered by the disc drive during seek operations. It is common for a disc drive to encounter read/write head oscillations during seek operations. To deal with this phenomena most servo systems incorporate "settle time" to allow time for the oscillations to dampen out prior to turning on the read gate.
Resonant modes found to be problematic for read/write head stability are typically the domain of mechanical interface integrity issues between the E-block and the cartridge bearing assembly of the actuator assembly. Dealing with E-block to cartridge bearing assembly mechanical interface issues requires establishment of a highly controllable assembly process and incorporation of a compliant, robust mechanical design.
Prior art techniques of coupling the E-block and the cartridge bearing assembly have mostly become obsolete due to the ever increasing performance demands placed on disc drives. Prior art coupling techniques have included press-fitting, threading engagement of the cartridge bearing assembly with the E-block, adhesive bonding, and mounting hardware that is passed through the E-block and treaded into the cartridge bearing assembly. Of these, adhesive bonding and mounting hardware are currently finding the greatest acceptance.
With adhesives, the ability to selectively position the E-block relative to the cartridge bearing assembly is a distinct advantage. However, problems include the potential contamination of the bearings by the adhesive and the difficulty of adjusting the position of the E-block relative to the cartridge bearing assembly once the adhesive has cured. With mounting hardware, the position of the E-block relative to the cartridge bearing assembly requires a specific alignment of the threaded apertures of the cartridge bearing assembly and the mounting apertures of the E-block.
A problem common to both coupling approaches occurs as a result of the holding force used to secure the components being concentrated along one side of the cartridge bearing assembly. When employing an adhesive to secure the cartridge bearing assembly to the E-block, failure of the adhesive to wick around and fill the gap between the cartridge bearing assembly and the E-block results in gaps that allow excessive read/write head movement. The source of this movement is the inability of the E-block to cartridge bearing mechanical interface to maintain the position of the E-block relative to the cartridge bearing assembly when a seek operation introduces a torsion into the system. The occurrence of movement during seek operations, specifically short seeks, results in the read/write head traveling beyond the data track being sought. The response of the servo system to this condition is to drive the heads in the opposite direction. In such situations it is common that resonances are set up in the actuator assembly in response to the energy poured into the system driving the read/write heads to oscillate.
An analogous condition occurs when mounting hardware used to attach the E-block to the cartridge bearing assembly is attached in a manner that results in holding forces being concentrated on one side of the cartridge bearing assembly. The combination of the components in conjunction with localized holding forces has been found to propagate resonant modes contrary to optimum performance of the disc drive.
Disc drive performance degradating resonant modes, linked to localization of holding forces, evidence a need for an improved cartridge bearing assembly and E-block coupling for actuator assemblies in disc drives.